The present invention relates to radiation imaging and specifically to a technology for suppressing degradation in quality of a reconstructed image due to variations in position of a radiation focal point.
As a representative of a radiation imaging apparatus which performs imaging using radiation, there has been known an X-ray CT (Computed Tomography) apparatus. A data acquisition system of the X-ray CT apparatus is principally includes an X-ray source and an X-ray detection section. The X-ray detection section further includes an X-ray detector and a collimator. The collimator includes a plurality of collimator plates provided so as to separate or partition X-ray detecting elements in the X-ray detector on their detection surface sides. See, for example, FIG. 16 of Japanese Unexamined Patent Publication No. 2010-005015.
The collimator is configured to remove scattered radiation to thereby prevent degradation of an output of each X-ray detecting element due to the scattered radiation and to obtain a distinct reconstructed image. If segments separated by collimator plates are not properly directed to an X-ray source, it may cause degradation of a reconstructed image. Therefore, the collimator plates are generally required to have very high accuracy of their installation.
There is no problem if the collimator plates form the segments in a completely ideal state. However, since the collimator used in the X-ray CT apparatus utilizes about 1000 collimator plates, for example, some small variations may actually occur.
Although the X-ray source that emits X-rays is ideally a point source, it actually has some width and the position thereof slightly changes depending on the environment. When the X-ray source is an X-ray tube, for example, a target of the X-ray tube is slightly deformed depending on a change in its temperature. Hence, the position and size of an X-ray focal point changes.
Due to variations in segments formed by the collimator plates and variations (focal movement) in the position of the X-ray focal point of the X-ray source, variations in segments in X-ray detecting elements are developed at a rate in a region on which X-rays are actually applied, to a region having an ability to receive X-rays by the X-ray detector. The variations direct noise that flows into the X-ray detector and thereby degrades the quality of a reconstructed image. Since the required accuracy is high even if the collimator would be fabricated under a fully managed environment, variations in the X-ray CT apparatus may not satisfy the accuracy required for the reconstructed image.
With such a situation, there has been a demand for a technology for correcting the effects of slight variations in the position of a radiation focal point to radiation projection data.